1. Field of the Invention
The present invention relates to vehicle-mounted electronic control devices, for example, engine control devices, transmission control devices, or the like. More particularly, the present invention relates to a vehicle-mounted electronic control device equipped with a switching power supply which steps down a power supply voltage fed from a vehicle-mounted battery to obtain an intermediate voltage, and a power supply control circuit unit which supplies a stabilization voltage with low electric power consumption and high accuracy to a control circuit unit including a microprocessor via a linearly controlled type constant voltage controlled power supply fed from the switching power supply.
2. Description of the Related Art
In a switching power supply (alternatively referred to as a switching regulator or a chopper regulator) which controls a current carrying duty of a switching element and obtains a predetermined stabilization voltage, low electric power consumption and a high power stabilization voltage can be obtained. However, ripple fluctuation of an output voltage due to switching operation of the switching element is generated; and therefore, a continuously controlled power supply (referred to as a linearly controlled power supply, a dropper power supply, a series regulator, or the like), which obtains a more highly accurate stabilization voltage by performing continuous control of a conductive state of a second switching element connected in series to the switching element, is subordinately connected. This technique has been widely and practically used.
In this case, when an input voltage of the continuously controlled power supply (output voltage of the switching power supply) is stable, a difference in voltage between input and output is small and thus electric power consumption is suppressed. However, a problem exists in that, when the input voltage is pulsative, the difference in voltage between the input and the output is large and therefore sufficient suppression of the electric power consumption cannot be performed.
In order to reduce pulsating components of the output voltage of the switching power supply, configuration may be such that inductance of a choke coil connected in series and capacitance of an output capacitor are made to increase to switch the switching element at high speed cycle; however, in this case, problems exist in that the choke coil and the output capacitor are increased in size and heat generation due to switching loss of the switching element is increased.
Furthermore, problems exist in that, in the case where a voltage drop of the switching element at the time when the switching element is conducted and driven, not only the heat generation of the switching element is increased, but also a target output voltage cannot be obtained when the power supply voltage of the vehicle-mounted battery is lowered and thus a minimum guaranteed voltage is increased and degraded.
For example, according to the following Patent Document 1, a switching power supply (chopper regulator) and an NPN type junction transistor serving as a continuously controlled power supply (series regulator), both of which are subordinately connected with each other, are used; and base current of each transistor is supplied from an external power supply which feeds to the switching power supply. Therefore, problems exist in that a base voltage of a former stage transistor is restricted to equal to or lower than a collector voltage and therefore a voltage drop between collector and emitter terminals at the time when the former stage transistor is conducted and driven is large; and accordingly, not only heat generation of the former stage transistor is increased, but also a target output voltage cannot be obtained when a power supply voltage is lowered and thus a minimum guaranteed voltage is increased and degraded.
Furthermore, according to the following Patent Document 2, in a switching power supply (first power supply regulator) and a continuously controlled power supply (second power supply regulator), both of which are subordinately connected with each other, the switching power supply on the former stage side uses a P channel type field effect transistor. In this case, a voltage drop between source and drain terminals at the time when the former stage transistor is completely conducted is small. However, a problem exists in that, in the case of the P channel type transistor, when the power supply voltage is lowered to equal to or lower than a load side voltage, a reverse conductive state of the transistor is generated and accordingly an undershoot is generated in a charging voltage of an output capacitor.
In order to prevent this problem, in the case of Patent Document 2, a diode for backflow prevention is connected in series to a source terminal of the transistor. Therefore, problems exist in that, even when the voltage drop between the source and drain terminals at the time when the former stage transistor is conducted and driven is small, a voltage drop including a voltage drop due to the diode for backflow prevention is large; and accordingly, not only heat generation of the former stage transistor including the diode is increased, but also a target output voltage cannot be obtained when the power supply voltage is lowered and thus a minimum guaranteed voltage is increased and degraded.
[Patent Document]
[Patent Document 1] Japanese Unexamined Patent Publication No. H07-095765 (FIG. 1, Abstract of the disclosure)
[Patent Document 2] Japanese Unexamined Patent Publication No. 2003-316482 (FIG. 1, Abstract of the disclosure)
According to a direct current stabilization power supply apparatus by Patent Document 1, even when reverse conduction of the switching element is generated, a problem does not exist because of high resistance; however, problems exist in that a forward voltage drop of the switching element is large, the heat generation is increased, and the minimum guaranteed voltage is increased and degraded.
Furthermore, according to a power supply circuit of Patent Document 2, problems exist in that, although reverse conduction of the switching element is possibly generated, the reverse conduction is prevented by the diode for backflow prevention, a forward voltage drop of the switching element including the diode is large, the heat generation is increased, and the minimum guaranteed voltage is increased and degraded.
Incidentally, in the case where reverse the conduction is generated in the switching element when the power supply voltage is abnormally lowered, charging electric charge of the output capacitor of the switching element is discharged to the upstream power supply side via a choke coil, current flowing through the choke coil is not steeply decreased even when voltages at both sides coincide, and discharging is maintained for an interim period; and accordingly, an undershoot in which a voltage at both ends of the output capacitor on the downstream side is more lowered than a voltage on the upstream power supply side is generated. As a result, a problem exists in that, despite that the power supply voltage is equal to or higher than the minimum guaranteed voltage, an actually obtained voltage is smaller voltage and a microprocessor driven by a constant voltage controlled power supply is erroneously operated.